This invention relates to a photo-lithography and, more particularly, to a photoacid generator, a chemically amplified resist containing the photoacid generator and a pattern transfer method.
A dynamic random access memory is a typical example of the ultra large scale integration. Growing research and development efforts are being made for the ultra large scale integration which will be improved in integration density and operation speed. One of the approaches for a high-speed high-integration density semiconductor integrated circuit device is to scale down the circuit components. The semiconductor integrated circuit devices are usually fabricated through epitaxial growing techniques and pattern transfer technologies. Submicron-order pattern transfer technologies are required for the miniature circuit components.
A photo-mask is exposed to light. Then, a pattern is transferred from a photo-mask to a photo-resist layer so as to form a latent image in the photo-resist layer. When the latent image is developed, the photo-resist layer is patterned into a photo-resist mask with a submicron-order pattern. The shorter the wavelength, the finer the transferred pattern. In general, the resolution R in an optical system is expressed by Rayleigh""s equation, i.e., R=kxc2x7xcex/NA where k is a process factor, xcex is the wavelength of the light and NA is the numerical aperture. The resolution is a function of the wavelength. It is understood from the Rayleigh""s equation that a miniature pattern is obtainable from an optical system at a small resolution. A short wavelength light is required for the optical system at a small resolution.
0.22 micron line-and-space patterns are used in a 256 mega-bit dynamic random access memory device. The 0.22 micron line-and-space pattern is transferred through a KrF excimer laser light. The wavelength of the KrF excimer laser light is 248 nanometers. Dynamic random access memory devices in the next generation, i.e., 1 mega-bit dynamic random access memory devices are to be fabricated by using 0.15 micron patterns or less. The wave-length of the KrF excimer laser light is too long to transfer the 0.15 micron patterns to photo-resist layers. ArF excimer laser light and F2 excimer laser light have the wavelengths shorter that that of the KrF excimer laser light. The wavelength of ArF excimer laser light is 193 nanometers, and the wave-length of F2 eximer laser light is 157 nanometers. However, it is said that far ultraviolet rays and vacuum ultraviolet rays will be required for the 0.15 micron patterns. Researchers are now developing the photo-lithography using the ArF excimer laser light, and reports are published by Donald C. Hofer in Jouinal of Photopolymer Science and Technology, vol. 9, No. 3, pages 387 to 397, 1996.
However, as well as the high resolution, a highly sensitive photo-resist is required for the photo-lithography using the ArF excimer laser light or F2 excimer laser light. This is because of the fact that the gas used for generating the laser light is short in lifetime. Moreover, the laser light is much liable to damage the lenses incorporated in the optical system.
In order to enhance the sensitivity of the photo-resist, a chemically amplified photo-resist is popular to the skilled persons. The chemically amplified photo-resist contains photoacid generator. The photoacid generator is a kind of photo-sensitive material, and accelerates the formation of latent images. A typical example of the chemically amplified photo-resist is disclosed in Japanese Patent Application laid-open No. 2-27660. The prior art chemically amplified photo-resist is composed of poly(p-tert-butoxycarbonyloxy-xcex1-methylstyrene) and photoacid generator. The photoacid generator is triphenylsulfonium hexafluoroarsenate. Hiroshi Ito and C. Grant Wilson report that the prior art chemically amplified photo-resist is widely used in the photo-lithography using the KrF excimer laser light (see American Chemical Society Symposium Series, vol. 242, pages 11-23, 1984).
When the chemically amplified photo-resist is exposed to the light, photoacid generator generates proton acid. After the pattern transfer through the exposure, the chemically amplified photo-resist is baked. Then, the proton acid gives rise to an acid-catalyzed reaction with the resist resin. By virtue of the acid-catalyzed reaction, the chemically amplified photo-resist achieves an extremely high sensitivity. The photo-reaction efficiency is defined as the amount of reaction per single photon. The standard photo-resist merely achieves the photo-reaction efficiency less than 1. However, the chemically amplified photo-resist achieves the photo-reaction efficiency drastically increased rather than the standard photo-resist. Most of the photo-resist presently developed are categorized in the chemically amplified photo-resist.
An example of the photoacid generator was developed by J. V. Crivello (see Journal of the Organic Chemistry, vol. 43, No. 15, pages 3055 to 3058, 1978). The photoacid generator is composed of the derivative of triphenylsulfonium salt, and is widely used for the chemically amplified photo-resist presently available.
The derivative of triphenylsulfonium salt is available for the chemically amplified photo-resist for the ArF excimer laser light lithography as reported by Nozaki et. al. in Journ al of Photopolymer Science and Technology, vol. 10, No. 4, pages 545 to 550, 1997 and by Yamachika et. al. in Journal of Photopolymer Science and Technology, vol. 12, No. 4, pages 553 to 560, 1990. However, the derivative of triphenylsulfonium salt strongly absorbs the rays equal in wavelength to or less than 220 nanometers. When the chemically amplified photo-resist containing the derivative of triphenylsulfonium salt is used in the photo-lithography using the ray equal in wavelength to or less than 220 nanometers as the exposure light, the derivative of triphenylsulfonium salt is causative of reduction in transparency of the chemically amplified photo-resist, and, accordingly, the resolution is lowered as reported by Takuya Naitoh et. al. in the proceedings of the 8th Lectures on Photo-Reactive Materials for Electric Devices, pages 16-18, 1999.
The photo-lithography using ArF excimer laser light is appropriate to the pattern transfer for extremely miniature patterns. For this reason, when the latent images are developed, the photo-resist mask has extremely narrow spaces, and the side surfaces defining the extremely narrow spaces are strongly influential in the uniformity of pattern. If the pattern edge roughness is poor, the ratio of the unevenness to the pattern width is large, and, accordingly, the uniformity of pattern becomes poor. Since the derivative of triphenylsulfonium salt absorbs the exposure light, the amount of exposure light is gradually reduced from the incident surface toward the back surface of the chemically amplified resist, and the contrast between the exposed portion and the non-exposed portion is made poor. This results in a low resolution and, accordingly, poor uniformity of the pattern. This is the problem inherent in the prior art chemically amplified photo-resist for the photo-lithography using the ultraviolet rays equal in wavelength to or less than 220 nanometers.
It is therefore an important object of the present invention to provide a photoacid generator, which keeps the transparency of resist resin high enough to achieve good pattern uniformity.
It is also an important object of the present invention to provide a chemically amplified photo-resist, which is transparent to 130-220 nanometer wavelength ultraviolet rays, high in photo-reaction efficiency, i.e., photoacid generating efficiency, high in resolution and low in pattern edge roughness.
It is another important object of the present invention to provide a pattern transfer method which is available for the ultra large scale integration in the next generation.
In accordance with one aspect of the present invention, there is provided a photoacid generator containing at least one first sulfonium salt compound selected from the group consisting of first sulfonium salt compounds expressed by general formula [1]
where each of R1 and R2 is straight chain, branching, monocyclic or cross-linked cyclic alkyl group, each of R3, R4, R5 and R6 is hydrogen atom, halogen atom, alkyl group having carbon number from 1 to 4 or alkoxyl group, X is xe2x80x94CH2xe2x80x94, xe2x80x94C2H2xe2x80x94 or xe2x80x94OCH2xe2x80x94 and Yxe2x88x92 is a counter ion, and
at least one second sulfonium salt compound selected from the group consisting of second sulfonium salt compounds expressed by general formula [2]
where R7 is alkylene group or 2-oxoalkylene group, R8 is straight chain, branching, monocyclic, polycyclic or cross-linked cyclic alkyl group having oxo group or straight chain, branching, monocyclic, polycyclic or cross-linked cyclic alkyl group without oxo group and Yxe2x88x92 is a counter ion, and at least one of R7 and R8 has the oxo group.
In accordance with another aspect of the present invention, there is provided a chemically amplified photo-resist comprising a resin having at least one acid decomposable group and changing solubility in alkaline solution through an acid decomposition of the at least one acid decomposable group, and a photoacid generator containing at least one first sulfonium salt compound selected from the group consisting of first sulfonium salt compounds expressed by general formula [1]
where each of R1 and R2 is straight chain, branching, monocyclic or cross-linked cyclic alkyl group, each of R3, R4, R5 and R6 is hydrogen atom, halogen atom, alkyl group having carbon number from 1 to 4 or alkoxyl group, X is xe2x80x94CH2xe2x80x94, xe2x80x94C2H4xe2x80x94 or xe2x80x94OCH2xe2x80x94 and Yxe2x88x92 is a counter ion and at least one second sulfonium salt compound selected from the group consisting of second sulfonium salt compounds expressed by general formula [2]
where R7 is alkylene group or 2-oxoalkylene group, R8 is straight chain, branching, monocyclic, polycyclic or cross-linked cyclic alkyl group having oxo group or straight chain, branching, monocyclic, polycyclic or cross-linked cyclic alkyl group without oxo group and Yxe2x88x92 is a counter ion, at least one of R7 and R8 having the oxo group.
In accordance with yet another aspect of the present invention, there is provided a pattern transfer method comprising the steps of a) forming a chemically amplified photo-resist layer on a target layer, the chemically amplified photo-resist comprising a resin having at least one acid decomposable group and increasing solubility in alkaline solution through an acid decomposition of the at least one acid decomposable group, and a photoacid generator containing at least one first sulfonium salt compound selected from the group consisting of first sulfonium salt compounds expressed by general formula [1]
where each of R1 and R2 is straight chain, branching, monocyclic or cross-linked cyclic alkyl group, each of R3, R4, R5 and R6 is hydrogen atom, halogen atom, alkyl group having carbon number from 1 to 4 or alkoxyl group, X is xe2x80x94CH2xe2x80x94, xe2x80x94C2H4xe2x80x94 or xe2x80x94OCH2xe2x80x94 and Yxe2x88x92 is a counter ion and at least one second sulfonium salt compound selected from the group consisting of second sulfonium salt compounds expressed by general formula [2]
where R7 is alkylene group or 2-oxoalkylene group, R8 is straight chain, branching, monocyclic, polycyclic or cross-linked cyclic alkyl group having oxo group or straight chain, branching, monocyclic, polycyclic or cross-linked cyclic alkyl group without oxo group and Yxe2x88x92 is a counter ion, at least one of R7 and R8 having the oxo group, b) exposing the chemically amplified photo-resist layer to light having a wavelength fallen within the range from 130 nanometers to 220 nanometers for producing a latent image therein, c) baking the chemically amplified photo-resist layer formed with the latent image, and d) developing the latent image.